U.S. patent application number 10/037423 was filed with the patent office on 2002-10-10 for code assignment algorithm for synchronous ds-cdma links with sdma using estimated spatial signature vectors.
Invention is credited to Ertel, Richard B., Giallorenzi, Thomas R., Hall, Eric K..
Application Number | 20020146060 10/037423 |
Document ID | / |
Family ID | 26714123 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020146060 |
Kind Code |
A1 |
Ertel, Richard B. ; et
al. |
October 10, 2002 |
Code assignment algorithm for synchronous DS-CDMA links with SDMA
using estimated spatial signature vectors
Abstract
A method is disclosed for operating a synchronous space division
multiple access, code division multiple access communications
system. The method operates, within a coverage area of a base
station (BS) or radio base unit (RBU) having a multi-element
antenna array, for estimating a SSV for individual ones of a
plurality of active subscriber stations (SSs) and assigns a
spreading code to a subscriber station (SS) that minimizes the
similarity of the determined SSVs of the SSs in a spreading code
set. A metric used to measure the similarity of the spatial
signature vectors of the SSs comprises the squared sum of the inner
products of same code SSs' SSV with a current SS's SSV. The step of
assigning includes calculating the magnitude of the squared inner
product of the SSVs of all pairs of active SSs; using the
calculated values for determining .xi..sub.n(c) for each spreading
code that is not already used some specified maximum number of
times; and assigning to a SS the spreading code with a minimum
.xi..sub.n(c).
Inventors: |
Ertel, Richard B.; (Sandy,
UT) ; Giallorenzi, Thomas R.; (Reverton, UT) ;
Hall, Eric K.; (Sandy, UT) |
Correspondence
Address: |
HARRINGTON & SMITH, LLP
4 RESEARCH DRIVE
SHELTON
CT
06484-6212
US
|
Family ID: |
26714123 |
Appl. No.: |
10/037423 |
Filed: |
October 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60243808 |
Oct 27, 2000 |
|
|
|
Current U.S.
Class: |
375/130 ;
375/E1.002 |
Current CPC
Class: |
H01Q 1/246 20130101;
H04L 1/0003 20130101; H04J 13/0074 20130101; H01Q 21/205 20130101;
H04L 1/0009 20130101; H04W 16/02 20130101; H04B 1/707 20130101;
H04J 13/16 20130101; H04W 16/12 20130101; H04B 2201/709709
20130101; H01Q 3/2617 20130101; H04B 2201/70703 20130101 |
Class at
Publication: |
375/130 |
International
Class: |
H04B 001/69 |
Claims
What is claimed is:
1. A method for operating a wireless communications system for
assigning system resources to users, comprising: within a coverage
area of a base station (BS) having a multi-element antenna array,
estimating a spatial signature vector (SSV) for individual ones of
a plurality of active subscriber stations (SSs); and assigning a
system resource to a subscriber station (SS) that minimizes the
similarity of the determined SSVs of the SSs sharing the system
resource.
2. A method as in claim 1, wherein a metric used to measure the
similarity of the spatial signature vectors of the SSs comprises
the squared sum of the inner products of the SSs' SSV, that share
the resource, with the current SS's SSV.
3. A method as in claim 1, wherein the step of assigning includes
calculating the magnitude of the squared inner product of the SSVs
of all pairs of active SSs; using the calculated values for
determining .xi..sub.n(c) for the resource; and assigning to a SS
the system resource having a minimum .xi..sub.n(c).
4. A method as in claim 1, and further comprising beamforming using
the multi-element antenna array so as to maximize the signal to
interference plus noise ratio (SNR) for a signal transmitted from a
first SS by steering a null towards a second potentially
interfering SS to minimize interference from the second SS.
5. A method as in claim 4, wherein the step of beamforming
comprises a step of receiving the signal received from the desired
SS, followed by a step of spatial filtering.
6. A method as in claim 4, wherein the step of beamforming
comprises steps of operating the SSs to obtain channel estimates
comprised of the path amplitude and phase from each of m antenna
elements and to use the m channel estimates as a spatial signature
vector, and from the spatial signature vectors received from a
plurality of same-code subscriber stations, computing antenna
element weight vectors.
7. A synchronous space division multiple access, code division
multiple access communications system, comprising a data processor
for estimating, within a coverage area of a radio base unit (RBU)
having a multi-element antenna array, a spatial signature vector
(SSV) for individual ones of a plurality of active subscriber
stations (SSs) and for assigning a spreading code to a subscriber
station (SS) that minimizes the similarity of the determined SSVs
of the SSs in a code set.
8. A system as in claim 7, wherein a metric used by said data
processor to measure the similarity of the spatial signature
vectors of the SSs comprises the squared sum of the inner products
of same code SSs' SSV with a current SS's SSV.
9. A system as in claim 7, wherein said data processor operates to
calculate the magnitude of the squared inner product of the SSVs of
all pairs of active Sss, uses the calculated values for finding
.xi..sub.n(c) for each spreading code that is not already used some
specified maximum number of times, and assigns to a SS the
spreading code with a minimum .xi..sub.n(c).
10. A system as in claim 7, wherein said data processor further
operates beamforming circuitry with said multi-element antenna
array so as to maximize the signal to interference plus noise ratio
(SINR) for a signal transmitted from a first SS by steering a null
towards a second same-code SS to minimize interference from the
second same-code SS.
11. A system as in claim 10, wherein said beamforming circuitry
comprises a despreader for despreading a signal received from SSs
and a spatial filter having an input coupled to an output of said
despreader.
12. A system as in claim 11, wherein for a case of independent
fading on each antenna element of said antenna array, said system
achieves a diversity gain of M, where M is equal to the number of
antenna elements of said antenna array.
13. A method for operating a synchronous space division multiple
access, code division multiple access communications system for
assigning spreading codes to users, comprising: within a coverage
area of a base station (BS) having a multi-element antenna array,
estimating a spatial signature vector (SSV) for individual ones of
a plurality of active subscriber stations (SSs); and assigning a
spreading code to a subscriber station (SS) that minimizes the
similarity of the determined SSVs of the SSs in a code set.
14. A method as in claim 13, wherein a metric used to measure the
similarity of the spatial signature vectors of the SSs comprises
the squared sum of the inner products of the same code SSs' SSV
with the current SS's SSV.
15. A method as in claim 13, wherein the step of assigning includes
calculating the magnitude of the squared inner product of the SSVs
of all pairs of active SSs; using the calculated values for
determining .xi..sub.n(c) for each spreading code that is not
already used some specified maximum number of times; and assigning
to a SS the spreading code with a minimum .xi..sub.n(c).
16. A method as in claim 13, and further comprising beamforming
using the multi-element antenna array so as to maximize the signal
to interference plus noise ratio (SINR) for a signal transmitted
from a first SS by steering a null towards a second same-code SS to
minimize interference from the second same-code SS.
17. A method as in claim 16 wherein the step of beamforming
comprises a step of despreading the signal received from the
desired SS, followed by a step of spatial filtering.
18. A method as in claim 16, wherein the step of beamforming
comprises steps of operating the SSs to obtain channel estimates
comprised of the path amplitude and phase from each of m antenna
elements and to use the m channel estimates as a spatial signature
vector, and from the spatial signature vectors received from a
plurality of same-code subscriber stations, computing antenna
element weight vectors.
19. A method for operating a code division multiple access
communications system, comprising: estimating a spatial signature
vector for individual ones of a plurality of active users located
within a coverage area of a base station that operates with a
multi-element antenna array; calculating the magnitude of the
squared inner product of the spatial signature vectors of pairs of
active users; using the calculated values of the magnitude of the
squared inner product of the spatial signature vectors for
determining a spatial signature vector similarity metric for
spreading codes not already in use some maximum number of times;
and assigning a spreading code to a user that minimizes the spatial
signature vector similarity metric.
Description
CLAIM OF PRIORITY FROM COPENDING PROVISIONAL PATENT APPLICATION
[0001] This patent application claims priority from U.S.
Provisional Patent Application No. 60/243,808, filed on Oct. 27,
2000, the disclosure of which is incorporated by reference herein
in its entirety.
FIELD OF THE INVENTION
[0002] These teachings relate generally to wireless communications
systems and methods, and relate in particular to techniques for
assigning pseudo-noise (PN) spreading codes to users in a
Synchronous Code Division Multiple Access (S-CDMA) system.
BACKGROUND OF THE INVENTION
[0003] In a synchronous direct-sequence code division multiple
access (S-CDMA) system, users communicate simultaneously using the
same frequency band via orthogonal modulation or spread spectrum.
The number of orthogonal spreading codes (>1) limits the total
capacity of the system. To increase the capacity of a CDMA system
in a given service area, without requiring additional frequency
bandwidth, space division multiple access (SDMA) can be
employed.
[0004] In S-CDMA systems a set of orthogonal DS-CDMA codes are
assigned to the cell of interest. However, the number of available
orthogonal codes for a given spreading factor is limited, resulting
in the capacity of the conventional S-CDMA system often being code
limited.
SUMMARY OF THE INVENTION
[0005] In accordance with an aspect of these teachings, a code
assignment algorithm is described for S-CDMA wireless
communications systems that utilizes SDMA to enhance system
capacity. The code assignment algorithm is applicable to both the
forward and the reverse channels.
[0006] The inventors have realized that when SDMA is used in
conjunction with S-CDMA it becomes possible to reuse code sequences
within the same cell, thereby providing an increase in system
capacity. Theoretically, with an M element antenna array receiver
it is possible to reuse each code sequence M times.
[0007] SDMA is optimally achieved by exploiting the differences in
spatial signature vectors (SSVs) of the various users in the cell.
In general, the greater the difference in the SSVs of users
assigned to a given spreading code, the greater is the SDMA
isolation of the users' signals. Since the performance of the SDMA
system is tightly linked to the spatial properties of the users
with the same code, care is taken to insure that the set of users
that are assigned an identical spreading code are spatially
compatible. A good code assignment scheme in accordance with these
teachings assigns identical codes to users having most dissimilar
spatial properties.
[0008] In the presently preferred embodiment of these teachings a
code assignment algorithm is based upon estimating the SSVs of
active users. The properties of the estimated SSVs are then
employed in an attempt to place users into spatially compatible
groups. For convenience, this code assignment algorithm may be
referred to herein as an SSV Based Code Assignment Algorithm
(SBCAA).
[0009] A method in accordance with these teachings operates to
assign system resources to users of a wireless communications
system. In the preferred embodiment the method operates, within a
coverage area of a base station (BS) or radio base unit (RBU)
having a multi-element antenna array, for estimating a SSV for
individual ones of a plurality of active subscriber stations (SSs)
and assigns a spreading code to a subscriber station (SS) that
minimizes the similarity of the determined SSVs of the SSs in a
spreading code set. A metric used to measure the similarity of the
spatial signature vectors of the SSs comprises the squared sum of
the inner products of same code SSs' SSV with a current SS's SSV.
The step of assigning includes calculating the magnitude of the
squared inner product of the SSVs of all pairs of active SSs; using
the calculated values for determining .xi..sub.n(c) for each
spreading code that is not already used some specified maximum
number of times; and assigning to a SS the spreading code with a
minimum .xi..sub.n(c), where .xi..sub.n(c) is a SSV similarity
metric.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The above set forth and other features of these teachings
are made more apparent in the ensuing Detailed Description of the
Preferred Embodiments when read in conjunction with the attached
Drawings, wherein:
[0011] FIG. 1 is simplified block diagram of a wireless access
reference model that pertains to these teachings;
[0012] FIG. 2 is block diagram of a physical (PHY) system reference
model showing a major data flow path;
[0013] FIG. 3 shows an Error Control Coding (ECC) and scrambling
technique for single CDMA channel;
[0014] FIG. 4 is a Table illustrating exemplary parameters for a
3.5 MHz RF channelization;
[0015] FIG. 5 is a Table depicting an aggregate capacity and
modulation factors versus modulation type and antenna array size
(number of elements);
[0016] FIGS. 6A-6H are mathematical expressions useful in
explaining the presently preferred embodiment of the use of spatial
signature vectors;
[0017] FIG. 7 is an illustration of SDMA for two users, wherein
antenna patterns are used to provide orthogonal channels to the
users;
[0018] FIG. 8 is a circuit diagram of a spatial filter for user
n;
[0019] FIG. 9 is a logic flow diagram illustrating the operation of
a SSV-based CDMA code assignment algorithm;
[0020] FIG. 10 is a graph showing SINR versus a cumulative
distribution function (cdf) for SSV-based code assignment, in
accordance with these teachings, and for a random code
assignment;
[0021] FIG. 11 is a graph showing the number of users supported
versus the required antenna array output SINR for the SSV-based
code assignment and for a random code assignment; and
[0022] FIG. 12 is a graph showing a number of users supported per
antenna element per code versus the required SINR for SSV-based
code assignment and for a random code assignment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] Disclosed herein is a physical (PHY) system intended for
IEEE 802.16 and related standards, although those having skill in
the art should realize that various aspects of these teachings have
wider applicability. The disclosed system is but one suitable
embodiment for practicing the teachings of this invention.
[0024] The PHY technique is based on a hybrid synchronous DS-CDMA
(S-CDMA) and FDMA scheme using quadrature amplitude modulation
(QAM) and trellis coding. For a general background and benefits of
S-CDMA with trellis-coded QAM one may refer to R. De Gaudenzi, C.
Elia and R. Viola, "Bandlimited Quasi-Synchronous CDMA: A Novel
Satellite Access Technique for Mobile and Personal Communication
Systems," IEEE Journal on Selected Areas in Communications, Vol.
10, No. 2, February 1992, pp. 328-343, and to R. De Gaudenzi and F.
Gianneti, "Analysis and Performance Evaluation of Synchronous
Trellis-Coded CDMA for Satellite Applications," IEEE Transactions
on Communications, Vol. 43, No. 2/3/4, February/March/April 1995,
pp. 1400-1409.
[0025] The ensuing description focuses on a frequency division
duplexing (FDD) mode. While a time division duplexing (TDD) mode is
also within the scope of these teachings, the TDD mode is not
discussed further.
[0026] What follows is an overview of the PHY teachings which will
be useful in gaining a fuller understanding of the teachings of
this invention.
[0027] The system provides synchronous direct-sequence code
division multiple access (DS-CDMA) for both upstream and downstream
transmissions. The system further provides spread RF channel
bandwidths from 1.75-7 MHz, depending on target frequency band, and
a constant chip rate from 1-6 Mcps (Million chips per second)
within each RF sub-channel with common I-Q spreading. The chip rate
depends on channelization of interest (e.g. 3.5 MHz or 6 MHz). The
system features orthogonal, variable-length spreading codes using
Walsh-Hadamard designs with spread factors (SF) of 1, 2, 4, 8, 16,
32, 64 and 128 chips/symbol being supported, and also features
unique spreading code sets for adjacent, same-frequency
cells/sectors. Upstream and downstream power control and upstream
link timing control are provided, as are single CDMA channel data
rates from 32 kbps up to 16 Mbps depending on SF (spreading factor)
and chip rate. In the preferred system S-CDMA channel aggregation
is provided for the highest data rates.
[0028] Furthermore, in the presently preferred embodiment FDMA is
employed for large bandwidth allocations with S-CDMA in each FDMA
sub-channel, and S-CDMA/FDMA channel aggregation is used for the
higher data rates. Code, frequency and/or time division
multiplexing is employed for both upstream and downstream
transmissions. Frequency division duplex (FDD) or time division
duplex (TDD) can be employed, although as stated above the TDD mode
of operation is not described further. The system features coherent
QPSK and 16-QAM modulation with optional support for 64-QAM.
End-to-end raised-cosine Nyquist pulse shape filtering is employed,
as is adaptive coding, using high-rate punctured, convolutional
coding (K=7) and/or Turbo coding (rates of 4/5, 5/6 and 7/8 are
typical). Data randomization using spreading code sequences is
employed, as is linear equalization in the downstream with possible
transmit pre-equalization for the upstream.
[0029] As will be described more fully below, also featured is the
use of space division multiple access (SDMA) using adaptive
beam-forming antenna arrays (e.g., 1 to 16 elements) at the base
station.
[0030] FIG. 1 shows the wireless access reference model per the
IEEE 802.16 FRD (see IEEE 802.16.3-00/02r4, "Functional
Requirements for the 802.16.3 Interoperability Standard."). Within
this model, the PHY technique in accordance with these teachings
provides access between one or more subscriber stations (SS) 10,
also referred to herein simply as users, and base stations (BS) 11
to support the user equipment 12 and core network 14 interface
requirements. An optional repeater 16 may be deployed. In the
preferred embodiment the BS 11 includes a multi-element adaptive
array antenna 11A, as will be described in detail below. The BS 11
may also be referred to herein as a Radio Base Unit (RBU).
[0031] In FIG. 2, the PHY reference model is shown. This reference
model is useful in discussing the various aspects of the PHY
technique. As is apparent, the SS 10 and BS transmission and
reception equipment may be symmetrical. In a transmitter 20 of the
BS 11 or the SS 10 there is an Error Control Coding (ECC) encoder
22 for incoming data, followed by a scrambling block 24, a
modulation block 26 and a pulse shaping/pre-equalization block 28.
In a receiver 30 of the BS 11 or the SS 10 there is a matched
filter/equalization block 32, a demodulation block 34, a
descrambling block 36 and an ECC decoder 38. These various
components are discussed in further detail below.
[0032] The PHY interfaces with the Media Access Control (MAC)
layer, carrying MAC packets and enabling MAC functions based on
Quality of Service (QoS) requirements and Service Level Agreements
(SLAs). As a S-CDMA system, the PHY interacts with the MAC for
purposes of power and timing control. Both power and timing control
originate from the BS 11, with feedback from the SS 10 needed for
forward link power control. The PHY also interacts with the MAC for
link adaptation (e.g. bandwidth allocation and SLAs), allowing
adaptation of modulation formats, coding, data multiplexing,
etc.
[0033] With regard to frequency bands and RF channel bandwidths,
the primary frequency bands of interest for the PHY include the
ETSI frequency bands from 1-3 GHz and 3-11 GHz as described in ETSI
EN 301 055, Fixed Radio Systems; Point-to-multipoint equipment;
Direct Sequence Code Division Multiple Access (DS-CDMA);
Point-to-point digital radio in frequency bands in the range 1 GHz
to 3 GHz, and in ETSI EN 301 124, Transmission and Multiplexing
(TM); Digital Radio Relay Systems (DRRS); Direct Sequence Code
Division Multiple Access (DS-CDMA) point-to-multipoint DRRS in
frequency bands in the range 3 GHz to 11 GHz, as well as with the
MMDS/MDS (digital TV) frequency bands. In ETSI EN 301 124, the
radio specifications for DS-CDMA systems in the fixed frequency
bands around 1.5, 2.2, 2.4 and 2.6 GHz are given, allowing
channelizations of 3.5, 7, 10.5 and 14 MHz. Here, the Frequency
Division Duplex (FDD) separation is specific to the center
frequency and ranges from 54 to 175 MHz. In ETSI EN 301 124,
Transmission and Multiplexing (TM); Digital Radio Relay Systems
(DRRS); Direct Sequence Code Division Multiple Access (DS-CDMA)
point-to-multipoint DRRS in frequency bands in the range 3 GHz to
11 GHz., the radio characteristics of DS-CDMA systems with fixed
frequency bands centered around 3.5, 3.7 and 10.2 GHz are
specified, allowing channelizations of 3.5, 7, 14, 5, 10 and 15
MHz. Here, FDD separation is frequency band dependant and ranges
from 50 to 200 MHz. Also of interest to these teachings are the
MMDS/ITSF frequency bands between 2.5 and 2.7 GHz with 6 MHz
channelizations.
[0034] With regard to multiple access, duplexing and multiplexing,
the teachings herein provide a frequency division duplex (FDD) PHY
using a hybrid S-CDMA/FDMA multiple access scheme with SDMA for
increased spectral efficiency. In this approach, a FDMA sub-channel
has an RF channel bandwidth from 1.75 to 7 MHz. The choice of FDMA
sub-channel RF channel bandwidth is dependent on the frequency band
of interest, with 3.5 MHz and 6 MHz being typical per the IEEE
802.16 FRD. Within each FDMA sub-channel, S-CDMA is used with those
users transmitting in the upstream and downstream using a constant
chipping rate from 1 to 6 Mchips/second. While TDD could be used in
a single RF sub-channel, this discussion is focused on the FDD mode
of operation. Here, FDMA sub-channel(s) are used in the downstream
while at least one FDMA sub-channel is required for the upstream.
The approach is flexible to asymmetric data traffic, allowing more
downstream FDMA sub-channels than upstream FDMA sub-channels when
traffic patterns and frequency allocation warrant. Based on
existing frequency bands, typical upstream/downstream FDMA channel
separation range from 50 to 200 MHz.
[0035] Turning now to the Synchronous DS-CDMA (S-DS/CDMA) aspects
of these teachings, within each FDMA sub-channel, S-CDMA is used in
both the upstream and the downstream directions. The chipping rate
is constant for all SS with rates ranging from 1 to 6 Mchips/second
depending on the FDMA RF channel bandwidth. Common I-Q spreading is
performed using orthogonal, variable-length spreading codes based
on Walsh-Hadamard designs, with spread factors ranging from 1 up to
128 chips per symbol (see, for example, E. Dinan and G. Jabbari,
"Spreading Codes for Direct Sequence CDMA and Wideband CDMA
Cellular Networks," IEEE Communications Magazine, September 1998,
pp.48-54. For multi-cell deployments with low frequency reuse,
unique spreading code sets are used in adjacent cells to minimize
interference.
[0036] An aspect of the preferred system embodiment is a symmetric
waveform within each FDMA sub-channel, where both the upstream and
downstream utilize the same chipping rate (and RF channel
bandwidth), spreading code sets, modulation, channel coding, pulse
shape filtering, etc.
[0037] Referring now to Code and Time Division Multiplexing and
channel aggregation, with a hybrid S-CDMA/FDMA system it is
possible to multiplex data over codes and frequency sub-channels.
Furthermore, for a given code or frequency channel, time division
multiplexing could also be employed. In the preferred approach, the
following multiplexing scheme is employed.
[0038] For the downstream transmission with a single FDMA
sub-channel, the channel bandwidth (i.e. capacity measured in
bits/second) is partitioned into a single TDM pipe and multiple CDM
pipes. The TDM pipe may be created via the aggregation of multiple
S-CDMA channels. The purpose of this partition is based on the
desire to provide Quality of Service (QoS). Within the bandwidth
partition, the TDM pipe would be used for best effort service (BES)
and for some assured forwarding (AF) traffic. The CDM channels
would be used for expedited forwarding (EF) services, such as VoIP
connections or other stream applications, where the data rate of
the CDM channel is matched to the bandwidth requirement of the
service.
[0039] The downlink could be configured as a single TDM pipe. In
this case a time slot assignment may be employed for bandwidth
reservation, with typical slot sizes ranging from 4-16 ms in
length. While a pure TDM downlink is possible in this approach, it
is preferred instead to employ a mixed TDM/CDM approach. This is so
because long packets can induce jitter into EF services in a pure
TDM link. Having CDMA channels (single or aggregated) dedicated to
a single EF service (or user) reduces jitter without the need for
packet fragmentation and reassembly. Furthermore, these essentially
"circuit-switched" CDM channels would enable better support of
legacy circuit-switched voice communications equipment and public
switched telephone networks.
[0040] For the upstream, the preferred embodiment employs a similar
partition of TDM/CDM channels. The TDM channel(s) are used for
random access, using a slotted-Aloha protocol. In keeping with a
symmetric waveform, recommended burst lengths are on the order of
the slot times for the downlink, ranging from 4-16 ms. Multi-slot
bursts are possible. The BS 11 monitors bursts from the SS 10 and
allocates CDMA channels to SSs upon recognition of impending
bandwidth requirements or based on service level agreements (SLAs).
As an example, a BS 11 recognizing the initiation of a VoIP
connection could move the transmission to a dedicated CDMA channel
with a channel bandwidth of 32 kbps.
[0041] When multiple FDMA sub-channels are present in the upstream
or downstream directions, similar partitioning could be used. Here,
additional bandwidth exists which implies that more channel
aggregation is possible. With a single TDM channel, data may be
multiplexed across CDMA codes and across frequency
sub-channels.
[0042] With regard now to Space Division Multiple Access (SDMA)
extensions, a further aspect of this multiple access scheme
involves the use of SDMA using adaptive beamforming antennas.
Reference can be made to J. Liberti and T. Rappaport, Smart
Antennas for Wireless CDMA, Prentice-Hall PTR, Upper Saddle River,
N.J., 1997, for details of beamforming with CDMA systems.
[0043] In the preferred embodiment the adaptive antenna array 11A
at the BS 11 is provided with fixed beam SS antennas. In this
approach the S-CDMA/FDMA channels can be directed at individual
SSs. The isolation provided by the beamforming allows the CDMA
spreading codes to be reused within the same cell, greatly
increasing spectral efficiency. Beamforming is best suited to CDM
rather than TDM channels. In the downstream, TDM would employ
beamforming on a per slot or burst basis, increasing complexity. In
the upstream, beamforming would be difficult since the BS 11 would
need to anticipate transmission from the SS in order to form the
beams appropriately. In either case, reuse of CDMA spreading codes
in a TDM-only environment would be difficult. With CDM, however,
the BS 11 may allocate bandwidth (i.e. CDMA channels) to the SS 10
based on need, or on SLAs. Once allocated, the BS 11 forms a beam
to the SS 10 to maximize signal-to-interference ratios. Once the
beam is formed, the BS 11 may allocate the same CDMA channel to one
or more other SSs 10 in the cell. It is theoretically possible for
the spectral efficiency of the cell to scale linearly with the
number of antennas in the BS array 11A.
[0044] SDMA greatly favors the approach of "fast circuit-switching"
over pure, TDM packet-switching in a CDMA environment. By "fast
circuit-switching", what is implied is that packet data services
are handled using dedicated connections, which are allocated and
terminated based on bandwidth requirements and/or SLAs. An
important consideration when providing effective packet-services
using this approach lies in the ability of the BS 11 to rapidly
determine bandwidth needs, and to both allocate and terminate
connections rapidly. With fast channel allocation and termination,
SDMA combined with the low frequency reuse offered by S-CDMA is a
preferred option, in terms of spectral efficiency, for FWA
applications.
[0045] A discussion is now made of waveform specifications. The
waveform includes the channel coding 22, scrambling 24, modulation
26 and pulse shaping and equalization functions 28 of the air
interface, as depicted in FIG. 2. Also included are waveform
control functions, including power and timing control. In the
presently preferred PHY, each CDMA channel (i.e. spreading code)
uses a common waveform, with the spreading factor dictating the
data rate of the channel.
[0046] With regard to the Error Control Coding (ECC) function 22 of
FIG. 2, the ECC is preferably high-rate and adaptive. High rate
codes are used to maximize the spectral efficiency of BWA systems
using S-CDMA systems that are code-limited. In code-limited
systems, the capacity is limited by the code set cardinality rather
than the level of the multi-user interference. Adaptive coding is
preferred in order to improve performance in multipath fading
environments. For the coding options, and referring as well to FIG.
3, the baseline code is preferably a punctured convolutional code
(CC). The constituent code may be the industry standard, rate 1/2,
constraint length 7 code with generator (133/171).sub.8. Puncturing
is used to increase the rate of the code, with rates of 3/4, 4/5,
5/6 or 7/8 supported using optimum free distance puncturing
patterns. The puncturing rate of the code may be adaptive to
mitigate fading conditions. For decoding (block 38 of FIG. 2), a
Viterbi decoder is preferred. Reference in this regard can be made
again to the above-noted publication R. De Gaudenzi and F.
Gianneti, "Analysis and Performance Evaluation of Synchronous
Trellis-Coded CDMA for Satellite Applications," IEEE Transactions
on Communications, Vol. 43, No. 2/3/4, February/March/April 1995,
pp. 1400-1409, for an analysis of trellis-coded S-CDMA.
[0047] Turbo coding, including block turbo codes and traditional
parallel and serial concatenated convolutional codes, are
preferably supported as an option at the rates suggested above. In
FIG. 3, the CC/Turbo coding is performed in block 22A, the
puncturing in block 22B, and the scrambling can be performed using
an XOR 24A.
[0048] Each CDMA channel is preferably coded independently.
Independent coding of CDMA channels furthers the symmetry of the
upstream and downstream waveform and enables a similar time-slot
structure on each CDMA channel. The upstream and downstream
waveform symmetry aids in cost reduction, as the SS 10 and BS 11
baseband hardware can be identical. The independent coding of each
S-CDMA/FDMA channel is an important distinction between this
approach and other multi-carrier CDMA schemes.
[0049] Randomization is preferably implemented on the coded bit
stream. Rather than using a traditional randomizing circuit, it is
preferred, as shown in FIG. 3, to use randomizing codes derived
from the spreading sequences used by the transmitting station.
Using the spreading codes allows different randomizing sequences to
be used by different users, providing more robust randomization and
eliminating problems with inter-user correlated data due to
periodic sequences transmitted (e.g. preambles). Since the
receiving station has knowledge of the spreading codes,
de-randomization is trivial. Randomization may be disabled on a per
channel or per symbol basis. FIG. 3 thus depicts the preferred
channel coding and scrambling method for a single CDMA channel.
[0050] With regard to the modulation block 26, both coherent QPSK
and square 16-QAM modulation formats are preferably supported, with
optional support for square 64-QAM. Using a binary channel coding
technique, Gray-mapping is used for constellation bit-labeling to
achieve optimum decoded performance. This combined coding and
modulation scheme allows simple Viterbi decoding hardware designed
for binary codes to be used. Differential detection for all
modulation formats may be supported as an option. Depending on the
channel coding, waveform spectral efficiencies from 1 to 6
information bits/symbol are realized.
[0051] The modulation format utilized is preferably adaptive based
on the channel conditions and bandwidth requirements. Both upstream
and downstream links are achievable using QPSK waveform provided
adequate SNR. In environments with higher SNR, up and downstream
links may utilize 16-QAM and/or 64-QAM modulation formats for
increased capacity and spectral efficiency. The allowable
modulation format depends on the channel conditions and the channel
coding being employed on the link.
[0052] In the preferred embodiment, end-to-end raised-cosine
Nyquist pulse shaping is applied by block 28 of FIG. 2, using a
minimum roll-off factor of 0.25. Pulse shape filtering is designed
to meet relevant spectral masks, mitigate inter-symbol interference
(ISI) and adjacent FDMA channel interference.
[0053] To mitigate multipath fading, a linear equalizer 32 is
preferred for the downstream. Equalizer training may be
accomplished using a preamble, with decision-direction used
following initial training. With S-CDMA, equalizing the aggregate
signal in the downlink effectively equalizes all CDMA channels.
Multipath delay spread of less than 3 .mu.s is expected for
Non-Line Of Sight (NLOS) deployments using narrow-beam
(10-20.degree.) subscriber station 10 antennas (see, for example,
J. Porter and J. Thweat, "Microwave Propagation Characteristics in
the MMDS Frequency Band," Proceedings of IEEE International Conf.
On Communications (ICC) 2000, New Orleans, La., USA, June 2000, and
V. Erceg, et al, "A Model for the Multipath Delay Profile of Fixed
Wireless Channels," IEEE Journal on Selected Areas in
Communications (JSAC), Vol. 17, No. 3, March 1999, pp. 399-410.
[0054] The low delay spread allows simple, linear equalizers with
8-16 taps that effectively equalize most channels. For the
upstream, pre-equalization may be used as an option, but requires
feedback from the subscriber station due to frequency division
duplexing.
[0055] Timing control is required for S-CDMA. In the downstream,
timing control is trivial. However, in the upstream timing control
is under the direction of the BS 11. Timing control results in
reduced in-cell interference levels. While infinite in-cell signal
to interference ratios are theoretically possible, timing errors
and reduction in code-orthogonality from pulse shape filtering
allows realistic signal to in-cell interference ratios from 30-40
dB. In asynchronous DS-CDMA (A-CDMA) systems, higher in-cell
interference levels exist, less out-of-cell interference can be
tolerated and higher frequency reuse is needed to mitigate
out-of-cell interference (see, for example, T. Rappaport, Wireless
Communications: Principles and Practice, Prentice-Hall PTR, Upper
Saddle River, N.J., 1996, pp. 425-431. The ability of
timing-control to limit in-cell interference is an important aspect
of achieving a frequency reuse of one in a S-CDMA system.
[0056] Power control is also required for S-CDMA systems. Power
control acts to mitigate in-cell and out-of-cell interference while
also ensuring appropriate signal levels at the SS 10 or the BS 11
to meet bit error rate (BER) requirements. For a SS 10 close to the
BS 11, less transmitted power is required, while for a distant SS
10, more transmit power is required in both the up and downstream.
As with timing control, power control is an important aspect of
achieving a frequency reuse of one.
[0057] Turning now to a discussion of capacity, spectral efficiency
and data rates, for a single, spread FDMA channel, the presently
preferred S-CDMA waveform is capable of providing channel
bandwidths from 1 to 16 Mbps. Using variable-length spreading
codes, each CDMA channel can be configured to operate from 32 kbps
(SF=128) to 16 Mbps (SF=1), with rates depending on the modulation,
coding and RF channel bandwidths. With S-CDMA channel aggregation,
high data rates are possible without requiring a SF of one. In
general, the use of S-CDMA along with the presently preferred
interference mitigation techniques enable the system to be
code-limited. Note, mobile cellular A-CDMA systems are always
interference-limited, resulting in lower spectral efficiency.
Recall also that in code-limited systems, the capacity is limited
by the code set cardinality rather than the level of the multi-user
interference. In a code-limited environment, the communications
channel bandwidth of the system is equal to the communications
channel bandwidth of the waveform, assuming a SF of one. In the
Table shown in FIG. 4 sample parameters are shown for a
hypothetical system using different coded modulation schemes and
assuming a code-limited DS-CDMA environment. The Table of FIG. 4
illustrates potential performance assuming a single 3.5 MHz channel
in both the upstream and downstream. The numbers reported apply to
both the upstream and downstream directions, meaning that upwards
of 24 Mbps full duplex is possible (12 Mbps upstream and 12 Mbps
downstream). With additional FDMA RF channels or large RF channels
(e.g. 6 MHz), additional communication bandwidth is possible with
the same modulation factors from the Table. As an example,
allocation of 14 MHz could be serviced using 4 FDMA RF channels
with the parameters described in the Table of FIG. 4. At 14 MHz,
peak data rates to a given SS 10 of up to 48 Mbps are achievable,
with per-CDMA channel data rates scaling up from 32 kbps. The
channel aggregation method in accordance with these teachings is
very flexible in servicing symmetric versus asymmetric traffic, as
well as for providing reserved bandwidth for QoS and SLA
support.
[0058] With regard to multi-cell performance, to this point both
the capacity and spectral efficiency have been discussed in the
context of a single, isolated cell. In a multi-cell deployment,
S-CDMA enables a true frequency reuse of one. With S-CDMA, there is
no need for frequency planning, and spectral efficiency is
maximized. With a frequency reuse of one, the total system spectral
efficiency is equal to the modulation factor of a given cell.
Comparing S-CDMA to a single carrier TDMA approach, with a typical
frequency reuse of 4, TDMA systems must achieve much higher
modulation factors in order to compete in terms of overall system
spectral efficiency. Assuming no sectorization and a frequency
reuse of one, S-CDMA systems can achieve system spectral
efficiencies from 1 to 6 bps/Hz, with improvements being possible
with SDMA.
[0059] While frequency reuse of one is theoretically possible for
DS-CDMA, the true allowable reuse of a specific deployment is
dependent on the propagation environment (path loss) and user
distribution. For mobile cellular systems, it has been shown that
realistic reuse factors range from 0.3 up to 0.7 for A-CDMA:
factors that are still much higher than for TDMA systems. In a
S-CDMA system, in-cell interference is mitigated by the orthogonal
nature of the S-CDMA, implying that the dominant interference
results from adjacent cells. For the fixed environments using
S-CDMA, true frequency reuse of one can be achieved for most
deployments using directional SS 10 antennas and up and downstream
power control to mitigate levels of adjacent cell interference. In
a S-CDMA environment, true frequency reuse of one implies that a
cell is code-limited, even in the presence of adjacent cell
interference.
[0060] For sectorized deployments with S-CDMA, a frequency reuse of
two is preferred to mitigate the interference contributed by users
on sector boundaries. In light of this reuse issue, it is
preferred, but not required, to use SDMA with adaptive beamforming,
rather than sectorization, to improve cell capacity. Since spectral
efficiency translates directly into cost, the possibility of a
frequency reuse of one is an important consideration.
[0061] The use of SDMA in conjunction with S-CDMA offers the
ability to dramatically increase system capacity and spectral
efficiency. SDMA uses the antenna array 11A at the BS 11 to
spatially isolate same code SSs 10 in the cell. The number of times
that a code may be reused within the same cell is dependent upon
the number of antenna elements in the array 11A, the array
geometry, the distribution of users in the cell, the stability of
the channel, and the available processing power. Theoretically, in
the absence of noise, with an M element antenna array 11 A it is
possible to reuse each code sequence M times, thereby increasing
system capacity by a factor of M. In practice, the code reuse is
slightly less than M due to implementation loss, frequency
selective multipath fading, and receiver noise. Regardless,
significant capacity gains are achievable with SDMA. With
appropriate array geometry and careful grouping of users sharing
CDMA codes, it is possible to achieve a code reuse of 0.9M or
better.
[0062] In an actual deployment the number of antenna elements of
the antenna array 11 A is limited by the available processing
power, the physical tower constraints, and system cost (e.g. the
number of additional RF front ends (RFFEs)). Selected array sizes
vary depending upon the required capacity of the given cell on a
cell-by-cell basis. The Table shown in FIG. 5 illustrates the
achievable aggregate capacity and modulation factor with typical
array sizes, assuming a code reuse equal to the number of antenna
elements. The aggregate capacity is defined as the total data rate
of the BS 11. Modulation factors exceeding 56 bps/Hz are achievable
with 64 QAM and a sixteen-element antenna array 11 A. It should be
noted that while SDMA increases the capacity of cell, it does not
increase the peak data rate to a given SS 10.
[0063] The PHY system disclosed herein is very flexible. Using
narrowband S-CDMA channels, the PHY system can adapt to frequency
allocation, easily handling noncontiguous frequency allocations.
The data multiplexing scheme allows great flexibility in servicing
traffic asymmetry and support of traffic patterns created by
higher-layer protocols such as the Transmission Control Protocol
(TCP) or the Real Time Protocol (RTP).
[0064] Deployments using the disclosed PHY are also very scalable.
When traffic demands increase, new frequency allocation can be
used. This involves adding additional FDMA channels, which may or
may not be contiguous with the original allocation. Without
additional frequency allocation, cell capacity can be increased
using the adaptive antenna array 11A and SDMA.
[0065] The high spectral efficiency of the disclosed waveform leads
to cost benefits. High spectral efficiency implies less frequency
bandwidth is required to provide a certain amount of capacity.
[0066] Using a symmetric waveform (i.e., a waveform that is the
same in the upstream and downstream directions) is a cost saving
feature, allowing the use of common baseband hardware in the SS 10
and the BS 11. The use of CDMA technology also aids in cost
reduction, as some CDMA technology developed for mobile cellular
applications may be applicable to gain economies of scale.
[0067] As a spread spectrum signal, the preferred waveform offers
inherent robustness to interference sources. Interference sources
are reduced by the spreading factor, which ranges from 1 to 128
(interference suppression of 0 to 21 dB.) At the SS 10,
equalization further suppresses narrowband jammers by adaptively
placing spectral nulls at the jammer frequency. Additional
robustness to interference is achieved by the directionality of the
SS antennas, since off-boresight interference sources are
attenuated by the antenna pattern in the corresponding direction.
At the BS 11, the antenna array 11A used to implement SDMA offers
the additional benefit of adaptively steering nulls towards
unwanted interference sources.
[0068] The presently preferred waveform exhibits several properties
that make it robust to channel impairments. The use of spread
spectrum makes the waveform robust to frequency selective fading
channels through the inherent suppression of inter-chip
interference. Further suppression of inter-chip interference is
provided by equalization at the SS 10. The waveform is also robust
to flat fading channel impairments. The adaptive channel coding
provides several dB of coding gain. The antenna array 11A used to
implement SDMA also functions as a diversity combiner. Assuming
independent fading on each antenna element, diversity gains of M
are achieved, where M is equal to the number of antenna elements in
the antenna array 11A. Finally, since the S-CDMA system is
code-limited rather than interference limited, the system may run
with a large amount of fade margin. Even without equalization or
diversity, fade margins on the order of 10 dB are possible.
Therefore, multipath fades of 10 dB or less do not increase the BER
beyond the required level.
[0069] The adaptive modulation also provides some robustness to
radio impairments. For receivers with larger phase noise, the QPSK
modulation offers more tolerance to receiver phase noise and filter
group delay. The adaptive equalizer at the SS 10 reduces the impact
of linear radio impairments. Finally, the use of clipping to reduce
the peak-to-average power ratio of the transmitter signal helps to
avoid amplifier saturation, for a given average power output.
[0070] An important distinction between the presently preferred
embodiment and a number of other CDMA approaches is the use of a
synchronous upstream, which allows the frequency reuse of one. Due
to some similarity with mobile cellular standards, cost savings are
possible using existing, low-cost CDMA components and test
equipment.
[0071] The presently preferred PHY is quite different from cable
modem and xDSL industry standards, as well as existing IEEE 802.11
standards. With a spreading factor of one chip/symbol, the PHY
supports a single-carrier QAM waveform similar to DOCSIS 1.1 and
IEEE 802.16.1 draft PHY (see "Data-Over-Cable Service Interface
Specifications: Radio Frequency Interface Specification",
SP-RF1v1.1-105-000714, and IEEE 802.16.1-00/01r4, "Air Interface
for Fixed Broadband Wireless Access Systems", September 2000.)
[0072] The presently preferred PHY technique provides an optimum
choice for IEEE 802.16A and for other applications. An important
aspect of the PHY is its spectral efficiency, as this translates
directly to cost measured in cost per line or cost per carried bit
for FWA systems. With a frequency reuse of one and efficient
support of SDMA for increased spectral efficiency, the combination
of S-CDMA with FDMA is an optimum technology for the fixed wireless
access market.
[0073] Benefits of the presently preferred PHY system include:
[0074] High spectral efficiency (1-6 bps/Hz system-wide), even
without SDMA;
[0075] Compatibility with smart antennas (SDMA), with system-wide
spectral efficiency exceeding 20 bps/Hz possible; and
[0076] A frequency reuse of one is possible (increased spectral
efficiency and no frequency planning).
[0077] The use of S-CDMA provides robustness to channel impairments
(e.g. multipath fading): robustness to co-channel interference
(allows frequency reuse of one); and security from
eavesdropping.
[0078] Also provided is bandwidth flexibility and efficiency
support of QoS requirements, flexibility to support any frequency
allocation using a combination of narrowband S-CDMA combined with
FDMA, while adaptive coding and modulation yield robustness to
channel impairments and traffic asymmetries.
[0079] The use of these teachings also enables one to leverage
mobile cellular technology for reduced cost and rapid technology
development and test. Furthermore, cost savings are realized using
the symmetric waveform and identical SS 10 and BS (RBU) 11
hardware.
[0080] Having thus described the overall PHY system, a more
detailed discussion will now be made of an aspect thereof that is
particularly pertinent to these teachings. More specifically, a
discussion will now be made of the presently preferred SSV Based
Code Assignment Algorithm (SBCAA). For the purposes of this
description it is assumed that the RBU 11 includes or has access to
a data processor that is capable of executing program steps that
implement the code assignment algorithm, as discussed in detail
below.
[0081] Discussing first the signal model, it is assumed that the
Radio Base Unit 11 is equipped with the above-described M element
antenna array 11 A. Let x(t) denote the received signal vector that
is observed at the antenna outputs at time t. For a multipath
channel, the analytic received signal vector component due to user
n is given by the expression shown in FIG. 6A, where L.sub.n is the
number of signal components and s.sub.n(t) is the analytic
transmitted signal of the n.sup.th user. For CDMA systems,
s.sub.n(t)=c.sub.n(t)b.sub.n(t)exp(-j.omega..sub.ct), where
c.sub.n(t) is the CDMA spreading code, b.sub.n(t) is the sequence
of data bits, and .omega..sub.c is the carrier frequency in radians
per second. The parameters .alpha..sub.nl, .tau..sub.nl and
.theta..sub.nl denote the complex amplitude, path delay and azimuth
angle of arrival of the l.sup.th path of the n.sup.th user,
respectively. The vector a(.theta..sub.nl) is the response vector
of the antenna array 11A in the direction of .theta..sub.nl.
Assuming a flat fading channel environment, x.sub.n(t) may be
expressed as x.sub.n(t)=v.sub.ns.sub.n(t), where the equation of
FIG. 6B shows the derivation of v.sub.n, which is defined as the
spatial signature vector (SSV) of user n. It is the SSVs of each of
the users that determine the performance of the SDMA system.
[0082] SDMA exploits the differences in the spatial characteristics
of the various users in the channel to provide nearly orthogonal
channels to the users. An illustration of SDMA is shown in FIG. 7.
The RBU 11 uses different effective beam patterns to isolate the
users' signals. In this case the antenna pattern of user 1 has a
null in the direction of user 2 and vice versa. In this way, the
two users are able to access the RBU 11 resources using the same
frequency, at the same time, and with the same spreading code,
without interfering with one another.
[0083] The effective antenna patterns of each user are generated by
summing amplitude scaled and phase rotated versions of the signals
observed at each antenna element, as is made more apparent in
diagram of the spatial filter 11B shown in FIG. 8. The antenna
array 11A output for user n may be expressed in accordance with the
mathematical expression shown in FIG. 6C, where y.sub.n(t) is the
output of the spatial filter 11B, w.sub.n,l is the complex weight
applied to the ith antenna element,. x.sub.l(t) is the signal
present on the ith channel, and * denotes complex conjugate. To
each user is applied a unique set of weights, thereby yielding
different effective antenna patterns. The process may be referred
to as beamforming or as spatial filtering.
[0084] It can be shown that the weight vector that maximizes the
output signal to interference plus noise ratio (SINR) is given by
w.sub.n=R.sub.it.sup.-1(n)v.sub.n, where v.sub.n is the spatial
signature of user n and R.sub.ll(n) is the corresponding
interference plus noise correlation matrix. The interference plus
noise correlation matrix R.sub.ll(n) may be expressed as shown in
the mathematical expression of FIG. 6D, wherein N denotes the
number of interfering signals, .sigma..sub.s.sup.2 is the variance
of the analytic received signal that is assumed to be equal for
each user due to power control, and .sigma..sub.n.sup.2 is the
variance of the additive white noise in the channel. The output
SINR present at the array 11A output corresponding to the optimum
weight vector is given by the expression shown in FIG. 6E.
[0085] The assignment of spreading codes has the potential to have
a major impact on the performance of the SDMA system. A presently
preferred algorithm for assigning CDMA codes to users is now
described. The algorithm attempts to minimize the similarity of the
SSVs of the users in each code set. A presently preferred metric
used to measure the similarity of the spatial signature vectors of
the users is the squared sum of the inner products of the same code
users' SSV with the current user's SSV.
[0086] To understand the motivation behind the use of this
presently preferred metric, consider the separation of two users.
In this case the optimum SINR equation presented in FIG. 6E may be
expressed in the form shown in FIG. 6F. For equal power users in a
line of sight (LOS) channel,
.parallel.v.sub.1.parallel..sup.2=.parallel.v.sub.2.parallel..sup.2=M,
where M is the number of antenna elements in the antenna array 11A.
In this case the optimum output SINR for both users can be
expressed as shown in the equation of FIG. 6G. Note that the
optimum output SINR is a function of the squared inner product of
v.sub.1 and v.sub.2. For convenience, let
.rho..sub.lJ=.vertline.v.sub.l.sup.Hv.sub.J.vertline..su- p.2. When
there are more than two users in the environment, then the optimum
output SINR is a function of just .rho..sub.iJ=.vertline.v.sub.l.-
sup.Hv.sub.J.vertline..sup.2 for each user, but is also dependent
upon the phase of the inner products of the SSVs. Regardless, it
can be shown through simulation that there is a negative
correlation between the output SINR and the quantity shown in the
equation of FIG. 6H, where S.sub.c denotes the set of users already
assigned to code c.
[0087] With regard to the estimation of the SSV, in the forward
link (BS 11 to SS 10) the SSV may be estimated using forward link
probing signals, as described by D. Gerlach and A. Paulraj,
Adaptive transmitting antenna arrays with feedback, IEEE Signal
Processing Letters, vol. 1, pp. 150-152, October 1994. In the
reverse link direction (SS 10 to BS 11) the SSV may be estimated
using known or estimated data transmitted by the SS 10. For the SS
10 that is transmitting on a non-shared PN code, the SSV maybe
estimated by despreading the signal on each antenna element. The
vector of the despread signal values is itself an estimate of the
SSV. To average over multiple symbols, the known or estimated phase
of the transmitted data is removed prior to averaging. For the SS
10 that is transmitting on a shared PN code, unique training data
is transmitted by each SS 10. The training patterns are designed
such that the same-code users are orthogonal (separable). The SSV
of each SS 10 is found by removing the known phase of the
transmitted data from the despread data, and then averaging over
the length of the training data. The average vector is itself an
estimate of the SSV. The estimated SSV, obtained by whatever means,
is then applied in the manner discussed above.
[0088] FIG. 9 is a logic flow diagram illustrating the operation of
the SSV-based CDMA code assignment algorithm in accordance with the
teachings of this invention.
[0089] Step A: Estimate the SSV of all active users.
[0090] Step B: Calculate the magnitude of the squared inner product
of the SSVs of all pairs of active users.
[0091] Step C: Using the values calculated in Step B, find
.xi..sub.n(c) for each code that is not already used some specified
maximum number of times, where .xi..sub.n(c) may be considered to
be a spatial signature vector similarity metric.
[0092] Step D. Assign to user k the code with the minimum
.xi..sub.n(c).
[0093] A simulation can be performed to compare the performance of
the presently preferred SSV-based code assignment algorithm with a
random assignment of codes to users. For example, assume that the
RBU 11 is equipped with a 16 element circular antenna array 11A
with an adjacent element spacing of five wavelengths, and that 120
codes are shared amongst the users. The total number of active
users is then 16.times.120=1920. Users are distributed in azimuth
according to a random uniform distribution over the range of
[0,2.pi.]. Only the LOS for each user is considered. It is also
assumed that power control is used on the reverse link (SS 10 to
RBU 11) such that all of the users are received with equal power.
The assumed SNR of the signal observed on each antenna element for
a given user after despreading is 15 dB. The cumulative
distribution function (cdf) of the array 11A output signal to the
interference plus noise ratio (SINR) is calculated from the SINR of
each user, over ten independent trials of user placements.
[0094] The cdf of the array output SINR is shown in the graph of
FIG. 10. The output SINR obtained by using the presently preferred
code assignment algorithm is between 3 to 5 dB higher than that
obtained through random code assignment. In FIG. 11 the number of
users are shown having an array output SINR that is greater than
the specified threshold level. For example, 1541 users have an
array output SINR that exceeds 15 dB using the presently preferred
code assignment algorithm, whereas only 954 have an array output
SINR that exceeds 15 dB using random code assignment. FIG. 12 shows
the number of users per antenna element per code with an output
SINR greater than the mantissa (a quantity having a good figure of
merit in which to compare the performance of antenna arrays with
differing numbers of antenna elements.)
[0095] It can thus be seen that by using the presently preferred
SSV-based code assignment algorithm approximately 1.6 times as many
users have an output SINR that exceeds the original input SINR, as
compared to randomly assigning spreading codes to users. The factor
of improvement is even greater when comparing the number of users
at higher output SINRs.
[0096] While described in the context of a S-CDMA system, it should
be appreciated that these teachings have applicability as well to
other types of wireless systems wherein users share system
resources, such as time slots and/or frequency channels. As such,
the teachings have applicability as well to, for example, TDMA and
FDMA systems. Furthermore, these teachings need not be limited to
synchronous wireless systems, as asynchronous wireless systems may
benefit as well from their use. Furthermore, while described in the
context of various exemplary modulation and channel coding formats,
frequencies, numbers of antenna elements, spreading factors, symbol
rates and the like, it should be realized that these are exemplary,
and are not to be construed in a limiting sense upon the practice
of this invention.
[0097] Thus, while these teachings have been particularly shown and
described with respect to preferred embodiments thereof, it will be
understood by those skilled in the art that changes in form and
details may be made therein without departing from the scope and
spirit of the invention described above.
* * * * *